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Scientists had fossil backbone backwards

13 January 2013, by Harriet Jarlett

Scientists have been able to document the exact structure of the backbone of the earliest four-legged animals, known as tetrapods, in an effort to understand how animals changed from moving in water, to moving on land.

Ichthyostega.

This new research may also have led to dramatic changes in our understanding of the early tetrapod fossil record.

Dr Stephanie Pierce from the Royal Veterinary College and Professor Jennifer Clack from the University of Cambridge, who led the international team in the NERC-funded investigation, have been studying a 360 million year old extinct animal, Ichthyostega; thought to be the first species to have made brief excursions onto land.

They had previously built up a 3D digital model of Ichthyostega's skeleton but on examination of the vertebral column, it became clear that something strange was happening. The researchers then scanned three fossil specimens using high-energy X-rays, produced by the ESRF synchrotron in Grenoble, in order to build extremely detailed 3D images of the spine. What they found was unexpected.

Until now, it was thought that early tetrapod's vertebrae were made up of four separate bones – one at the front, one above and a pair at the back. But, the images showed that in Ichthyostega's vertebrae the bones at the back have become fused to the one in front.

This discovery meant that what was previously thought to be the first bone in the series of each vertebra was, in reality, the last. 'In evolutionary terms this discovery is extremely important. Imagine putting the forelimb of an animal where the hindlimb should be; this mistaken arrangement would completely change how the animal would move. The same principle holds true for bones in the spine,' says Pierce.

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The finding follows more than a century of misunderstanding. Where early tetrapod fossils had become broken over time, the vertebrae were often reconstructed based on the assumption that the animals would look similar to those from much later in the fossil record. But the researchers have now shown that this interpretation is incorrect. Pierce explains: ' Ichthyostega made us open our minds, stand back and reassess the anatomy of other early tetrapod fossils. When we did this, it was obvious that the bones of their spines were also in the reverse order than what had previously been described.'

Finding a similar structure in many early tetrapods means that scientists may need to re-examine other key fossils to ensure that the evolution of the backbone is properly documented.

' Ichthyostega made us open our minds, stand back and reassess the anatomy of other early tetrapod fossils.' Dr Stephanie Pierce, Royal Veterinary College

On top of this potentially revolutionary discovery, the team also found the first evidence of a prehistoric sternum – a series of bones extending down the middle of the Ichthyostega's chest. This unexpected find adds weight to the group's previous work, which suggested that instead of walking on all four limbs, Ichthyostega preferred to drag its body along by its front legs, like a seal or a mudskipper. 'A sternum provides extra support along the ribcage, and is further evidence that the animal may have put a lot of weight on its chest while it moved,' says Clack.

The scientists hope to take this new information and, by combining it with earlier data about the animal's limbs, use it to build a picture of how the spinal column affected Ichthyostega's movement.